(1) Field of the Invention
The present invention generally relates to a suspension control system, and more particularly to a suspension control system for controlling a damping force of a variable damping force type shock absorber on the basis of a running condition of a vehicle.
(2) Description of the Related Art.
There is known a suspension control system in which the damping force of a shock absorber is controlled on the basis of the rate of change of the damping force. When the change rate of the damping force exceeds a predetermined value, that is, when the change rate changes abruptly due to a rough road surface or braking, the damping force with respect to a movement of the shock absorber is rapidly changed to a small level, so that the control responsibility of the shock absorber is improved.
There is also known a suspension control system in which an adjustment reference value provided for obtaining the damping force change rate used for changing the setting of the damping force is altered on the basis of a driving condition, such as a vehicle speed, so that ride comfort is improved (see Japanese Laid-Open Patent Application No. 64-67407).
The conventional suspension control system proposed in the above-mentioned Japanese Application presents an advantage in that the damping force is rapidly changed in accordance with the road surface condition so that good ride comfort is obtained. However, there is room for improvement in ride comfort in a case where the vehicle is continuously running on a flat road surface or a rough road surface for a predetermined time or longer. When the vehicle is continuously running on a flat road surface, the shock absorber is controlled so that the damping force thereof is maintained at a large level and thus the suspension is maintained in a hard state. In this state, the driver or passenger will feel the existence of a small roughness in the road surface. On the other hand, if the vehicle is continuously running on a rough road surface, the shock absorber is controlled so that the damping force thereof is continuously maintained at a small level and thus the suspension is continuously maintained in a soft state. This state deteriorates the road holding ability of the vehicle and causes the driver to feel uneasy.